Yazar "Korkmaz, Habip Gokay" seçeneğine göre listele

Listeleniyor 1 - 5 / 5
  • Küçük Resim Yok
    Öğe
    A novel approach to enhance formability in Ti-6Al-4V alloy: Experimental investigations and microstructural analysis of pulsating tensile test
    (Elsevier, 2024) Korkmaz, Habip Gokay; Yapan, Yusuf Furkan; Toros, Serkan; Turkoz, Mevlut
    Ti-6Al-4V alloy, widely utilized in aerospace, medical industries, and specialized applications, boasts exceptional properties. However, its limited formability poses challenges in manufacturing processes. The pulsating loading method emerges as a promising solution to enhance formability in such materials. This study delves into the impact of stress relaxation and loading-unloading tests on the formability of Ti-6Al-4V alloy, conducting tensile tests on sheets of two different thicknesses: 0.5 mm and 2.65 mm. Investigating parameters such as pulse starting strain, relaxation time, and strain increment in stress relaxation experiments, as well as unloading ratio and strain increment in loading-unloading experiments, enabled a comprehensive comparison of the two pulsating loading methods across different sheet thicknesses. Results indicate a notable increase in material formability, up to approximately 20 % for the 2.65 mm thickness and up to 50 % for the 0.5 mm thickness compared to monotonic loading. Stress relaxation time emerged as the most influential parameter for both thicknesses. Additionally, XRD analysis was employed to elucidate the microstructural reasons behind the observed formability enhancement, while SEM imaging provided insights into the fracture surface morphology. This systematic approach sheds light on the microstructural mechanisms underlying the effect of pulsating loading on material behavior.
  • Küçük Resim Yok
    Öğe
    Engineering solid oxide fuel cell electrode microstructure by a micro-modeling tool based on estimation of TPB length
    (Pergamon-Elsevier Science Ltd, 2021) Timurkutluk, Bora; Altan, Tolga; Toros, Serkan; Genc, Omer; Celik, Selahattin; Korkmaz, Habip Gokay
    In this study, a typical solid oxide fuel cell (SOFC) electrode microstructure is numerically optimized in terms of the volume fraction of the catalyst, electrolyte and pore phases via a novel tool based on Dream.3D for the synthetic microstructure reconstruction and COM-SOL Multiphysics (R) Modeling for visualizing and computing three/triple phase boundaries (TPBs). First, the properties of the representative volume element are studied by a parameter independence analysis based on the average particle size. The results indicate that the size of the representative volume element should be at least 10 times greater than the largest average particle size in the microstructure, while the number of mesh elements should be selected such that the smallest average particle size in the system is divided into at least 5. The method is then validated with the available studies in the literature and seems to agree well. Therefore, numerical reconstruction of SOFC electrodes by the pro-posed method is found to be a very useful tool in the viewpoints of accuracy, flexibility and cost. Finally, SOFC electrode microstructures having the same particle size distribution of an average particle size of 0.5 mm for each phase but with various phase volume fractions are generated and the resultant TPBs are computed similarly. It is found out that the volume fraction of each phase should be close to each other as much as possible to maximize the active TPB density and among the cases considered, the highest active TPB density of 9.53 mm/mm(3) is achieved for an SOFC electrode including 35 vol% catalyst, 35 vol % electrolyte and 30 vol% porosity. The active TPB density is also found to be around 93% of the total TPB density. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
  • Küçük Resim Yok
    Öğe
    Enhancing formability of Ti-6Al-4V cylindrical cups by pulsating hydroforming process: Experimental, numerical and microstructural investigations
    (Elsevier - Division Reed Elsevier India Pvt Ltd, 2024) Ozturk, Osman; Aydin, Mevlut; Gokcepinar, Omer Faruk; Ilbeyli, Harun Mert; Korkmaz, Habip Gokay; Yapan, Yusuf Furkan; Dilmec, Murat
    Ti-6Al-4V alloy sheet is an engineering material that is widely used due to its superior properties such as high strength-to-density ratio besides high temperature and corrosion resistance. However, its low formability at room temperature limits its wider applications. In this study, a cylindrical cup was hydroformed using a female die to examine how the pulsating effect would result under frictional conditions. Initially, finite element simulations were performed to design a proper die geometry. Next, forming tests were run on Ti-6Al-4V blanks under pressure increased monotonically and with pulsation, and microstructural analyses were performed on the formed specimens. The effects of pulsation frequency, amplitude, and base pressure on the formability were investigated. The nose radius/thickness ratio, maximum thinning, bursting pressure, and die-filling ratio measured on the specimens formed under monotonic and pulsating loadings were compared, and the improvement in the formability was demonstrated. An increase of 38.5 % in bursting pressure occurred and the nose radius of the part was decreased up to 30 % with pulsating loading. The die-filling ratio was improved from 87.9 % to 95.3 % with optimized pulsation parameters. The underlying microstructural reasons for the improved formability were elaborated using XRD, SEM, and TEM analyses.
  • Küçük Resim Yok
    Öğe
    Estimation of microscale redox tolerance for Ni-based solid oxide fuel cell anodes via three-dimensional finite element modeling
    (Pergamon-Elsevier Science Ltd, 2023) Altan, Tolga; Celik, Selahattin; Toros, Serkan; Korkmaz, Habip Gokay; Timurkutluk, Bora
    Reduction-oxidation (redox) cycles of Ni-based anodes in solid oxide fuel cells (SOFCs) directly affect the cell performance due to breaking anode three/triple phase boundary (TPB) networks at microscale. Furthermore, these microcracks accumulate with the number of redox cycles leading to mechanical damage in the cell as a result of continuous volumetric changes during the inevitable cyclic reduction and oxidation of the nickel oxide and nickel, threatening the service life of SOFC systems. Therefore, the redox process needs to be investigated as a phenomenon at microscale to understand and minimize its effects. In this regard, we suggest a microscale approach for the redox process of Ni-based SOFC anodes in this study. For this purpose, SOFC anode microstructures with different compositions and porosities are synthetically generated by Dream.3D software and me-chanical damages due to the redox cycle are investigated via element deletion through LS-DYNA for the first time in the literature. The anodes are characterized by computing the redox tolerance based on the resultant damage and the anode composition showing the highest redox tolerance is determined among the cases considered.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
  • Küçük Resim Yok
    Öğe
    Experimental and numerical investigation of the damage state of Ti-6Al-4V alloy sheet in the tensile test, hydraulic bulging, and hydroforming processes
    (Springer London Ltd, 2024) Yapan, Yusuf Furkan; Korkmaz, Habip Gokay; Toros, Serkan; Turkoz, Mevlut
    There has not been any damage prediction using Johnson-Cook's (JC) hardening and damage model in the hydraulic bulging (HB) and hydroforming (HF), which are the advanced manufacturing processes, of the Ti-6Al-4V (Ti64) alloy. In the presented study, the damage behavior of the Ti64 alloy sheet in the HB and HF processes was investigated both experimentally and numerically for the first time to address the existing research gap. In this context, firstly, tensile tests (TT) were carried out on samples with different stress triaxiality values at three different tensile speeds, and the fracture morphologies of the samples were examined to evaluate whether it was appropriate to use the JC hardening and damage model. Since the fracture surfaces generally exhibit a ductile fracture morphology and are affected by stress triaxiality and strain rate, it was determined that it would be appropriate to use the JC hardening model and damage criterion to predict the damage of the Ti64 alloy in finite element analysis (FEA). Then, JC model parameters were determined by fitting the stress-strain curve obtained from the FEA and experimental tensile tests. In the HB experiments, bulging height and thickness thinning were predicted by FEA with an accuracy of 97% and 96.85%, respectively. In the HF experiments, the experimental burst pressure, die inlet radius, and base radius were predicted correctly at a rate of 92.5%, 95.5%, and 97.8%, respectively. Also, the thickness of the sample showed good agreement with the FEA results. The fracture zones in each process exhibited good agreement with the experimental results. Thus, it has been demonstrated that the JC damage criterion can be successfully applied in FEA if the Ti64 titanium alloy is damaged in various processes.